Variable area nozzle for gas turbine engines driven by shape memory alloy actuators

ABSTRACT

A gas turbine engine includes a variable area nozzle having a plurality of flaps. The flaps are actuated by a plurality of actuating mechanisms driven by shape memory alloy (SMA) actuators to vary fan exist nozzle area. The SMA actuator has a deformed shape in its martensitic state and a parent shape in its austenitic state. The SMA actuator is heated to transform from martensitic state to austenitic state generating a force output to actuate the flaps. The variable area nozzle also includes a plurality of return mechanisms deforming the SMA actuator when the SMA actuator is in its martensitic state.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application relates to an application entitled “Shape MemoryAlloy Bundles and Actuators”, having an Attorney Docket No.EH-10350-3/3309P-110, filed on the same date herewith and assigned to acommon assignee.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] This invention relates to gas turbine engines and, moreparticularly, to variable area nozzles driven by shape memory alloyactuators therefor.

[0004] 2. Background Art

[0005] Conventional gas turbine engines include a fan section and a coreengine with the fan section having a larger outer diameter than that ofthe core engine. The fan section and the core engine are disposedsequentially about a longitudinal axis and are enclosed in a nacelle. Anannular path of primary airflow passes through the fan section and thecore engine to generate primary thrust. An annular path of duct or fanflow, disposed radially outward of the primary airflow path, passesthrough the fan section and exits through a fan nozzle to generate fanthrust.

[0006] The fan nozzles of conventional gas turbine engines have fixedgeometry. The fixed geometry fan nozzles must be suitable for take-offand landing conditions as well as for cruise conditions. However, therequirements for take-off and landing conditions are different fromrequirements for the cruise condition. For cruise conditions, it isdesirable to have a smaller diameter fan nozzle for increasing cruiseperformance and for maximizing fuel efficiency, whereas, for take-offand landing conditions, smaller diameter fan nozzles may cause an enginestall. Therefore, in many conventional engines, the cruise performanceand fuel efficiency are often compromised to ensure safety of the gasturbine engine at take-off and landing.

[0007] Some gas turbine engines have implemented variable area nozzles.The variable area nozzles have the ability of having a smaller fan exitnozzle diameter during cruise conditions and a larger fan exit nozzlediameter during take-off and landing conditions. The existing variablearea nozzles are either hydraulically or pneumatically actuated andresult in complex mechanisms that require extensive maintenance. Mostcommercial aircraft prefer to avoid additional maintenance. Furthermore,the existing variable area nozzle mechanisms add significant weight tothe engine. As is well known in the art, the extra weight adverselyeffects the overall performance of the aircraft. The additional weightreduces aircraft range and can result in additional fuel consumption foroperation of the engine. Therefore, it is critical in gas turbine enginefabrication to avoid a weight increase, since the weight increaseresulting from the addition of a variable area nozzle typically negatesbenefits gained from improved fuel efficiency resulting from the reduceddiameter of the variable area nozzle during cruise conditions. Thus,although variable area nozzles have been introduced into some gasturbine engines, the use of existing variable area nozzles on mostaircraft is not practical.

[0008] Therefore, it is desirable to develop a variable area nozzle thatdoes not require extensive maintenance and does not add significantweight to the gas turbine engine.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a gas turbineengine with a variable area nozzle that does not significantly increasethe overall weight of the gas turbine engine and does not requireextensive maintenance.

[0010] According to the present invention, a gas turbine engine includesa variable area nozzle having a plurality of flaps actuated by aplurality of actuating mechanisms driven by shape memory alloy (“SMA”)actuators to vary fan exit nozzle area. Each actuating mechanismincludes a four bar linkage with a drive arm engaged by the SMAactuator. The SMA actuator has a deformed shape in its martensitic stateand a parent shape in its austenitic state. The SMA actuator is heatedto transform from martensitic state to austenitic state generating aforce output to actuate the flaps. The SMA actuator is allowed to coolor is actively cooled to transform from austenitic state to martensiticstate.

[0011] The variable area nozzle also includes a plurality of returnmechanisms deforming the SMA actuator when the SMA actuator is in itsmartensitic state. In one embodiment of the present invention, thereturn mechanism is spring actuated. In another embodiment of thepresent invention, the return mechanism comprises a secondary SMAactuator engaging the drive arm of the four bar linkage.

[0012] According to one aspect of the present invention, the SMAactuator engages a four bar linkage to generate a sweeping motion.Depending on the configuration of the four bar linkage, the motion canbe either parallel or non-parallel.

[0013] One major advantage of the present invention is that the SMAactuated variable area nozzle does not require complex mechanismsnecessitating extensive maintenance.

[0014] Another major advantage of the present invention is that the SMAactuated variable area nozzle does not add significant weight to theengine, thereby improving the benefits associated with the overall fuelefficiency increase of the gas turbine engine.

[0015] The foregoing and other advantages of the present inventionbecome more apparent in light of the following detailed description ofthe exemplary embodiments thereof, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a partially broken away, simplified representation of agas turbine engine with a variable area nozzle;

[0017]FIG. 2 is an enlarged, simplified representation of the variablearea nozzle of FIG. 1 in the diverged position, driven by a SMAactuator, according to the present invention;

[0018]FIG. 3 is a simplified representation of the variable area nozzleof FIG. 2 in the converged position;

[0019]FIG. 4 is an enlarged, simplified representation of a four barlinkage of the variable area nozzle of FIG. 2 driven by the SMAactuator;

[0020]FIG. 5 is an enlarged, fragmentary, perspective view of the SMAactuator of FIG. 4;

[0021]FIG. 6 is a simplified, side elevation of the four bar linkageengaged by the SMA actuator of FIG. 4 taken along line 6-6;

[0022]FIG. 7 is an enlarged, simplified representation of the variablearea SMA actuator and nozzle of FIG. 1 with an actuating mechanismdriven by a primary SMA actuator and a return mechanism driven by asecondary SMA actuator, according to another embodiment of the presentinvention;

[0023]FIG. 8 is a simplified, side elevation of the actuating and returnmechanisms engaged by the primary and secondary SMA actuators of FIG. 7taken along line 8-8;

[0024]FIG. 9 is a schematic representation of another embodiment of afour bar linkage actuated by the SMA actuator of FIG. 5, with the SMAactuator being in a deformed state;

[0025]FIG. 10 is a schematic representation of the four bar linkage ofFIG. 9, with the SMA actuator being in a parent state; and

[0026]FIG. 11 is a schematic representation of a side elevation of thefour bar linkage of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027] Referring to FIG. 1, a gas turbine engine 10 includes a fansection 12 and a core engine 14 disposed sequentially about alongitudinal axis 16. The core engine 14 and the fan section 12 areenclosed in a nacelle 18. An annular path of primary airflow 20 passesthrough the fan section 12 and the core engine 14, generating primarythrust 22. An annular path of fan flow 24, disposed radially outward ofthe path of the primary air flow 20, bypasses the core engine 14 andflows through the fan section 12, generating fan thrust 26.

[0028] A variable area nozzle 30, shown in greater detail in FIGS. 2 and3, is disposed in a downstream portion 32 of the nacelle 18 with atrailing edge 34 of the nacelle 18 defining a fan exit nozzle area 36.Referring to FIGS. 2 and 3, the variable area nozzle 30 includes aplurality of flaps 38, circumscribing the core engine 14, a plurality ofactuating mechanisms 40, and a plurality of return mechanisms 42, witheach actuating mechanism 40 and each return mechanism 42 correspondingto each flap 38. Each of the plurality of the flaps 38 includes anaerodynamically shaped body 44 with a flap tip 46, coinciding with thetrailing edge 34 of the nacelle 18, and a flap actuating portion 48,extending upstream into the nacelle 18. The flaps 38 have a divergedposition and a converged position. In the diverged position, shown inFIG. 2 and in phantom in FIG. 3, the flaps 38 define an opened orenlarged fan exit nozzle area 36. In the converged position, shown inFIG. 3 and in phantom in FIG. 2, the flaps 38 are driven by theactuating mechanisms 40 to define a closed or reduced fan exit nozzlearea 36.

[0029] Referring to FIG. 4, each actuating mechanism 40 includes a fourbar linkage 52 comprising a drive arm 54 pivotably connected by means ofpivots 56, 58 to the flap actuating portion 48 and to a link 60,respectively. The four bar linkage 52 also includes a fixed member 62that extends from the nacelle 18. The flap actuating portion 48 and thelink 60 are pivotably attached to the fixed member 62 by means of pivots64, 66, respectively. Pivot 64 functions as a hinge for the flap 38. Theactuating mechanism 40 also includes a SMA actuator 68 engaging thedrive arm 54 of the four bar linkage 52. In the preferred embodiment ofthe present invention, the SMA actuator 68 comprises a plurality ofstrands 70 forming an array 72.

[0030] Referring to FIG. 5, each strand 70 is formed from a plurality ofSMA wires 74. In one embodiment of the present invention, each strand 70includes a center wire 76 surrounded by six wires 74 forming a secondrow and twelve wires 74 forming a third row. Each SMA array 72 includesa first end 78 and a second end 80. The SMA actuator 68 also includes acontrol system 81 for providing heat input and at least two endterminations 82 formed at the first and second ends 78, 80 of the array72, as shown in FIG. 6. The control system 81 communicates with the ends78, 80 of the SMA array 72. In the preferred embodiment, a single strand70 is looped to form the array 72 of fourteen (14) strands wide. Asingle strand formed into multiple passes lowers cost for endtermination and provides improved consistency. However, the array 72 canbe formed from multiple strands 70.

[0031] Referring to FIG. 6, the SMA actuator 68 is maneuvered among theplurality of the actuating mechanisms 40 such that the SMA actuator 68alternatingly engages the drive arms 54 of each actuating mechanism 40and a rib 83 separating the actuating mechanisms 40. The SMA actuator 68has a parent shape and a deformed shape. In one embodiment, shown inFIG. 3, the parent shape of the SMA actuator 68, corresponds to theclosed or converged position of the flaps 38, and the deformed shape ofthe SMA actuator 68, shown in FIG. 2, corresponds to open or divergedposition of the flaps 38.

[0032] Referring to FIG. 2, each return mechanism 42, according to oneembodiment of the present invention, comprises a spring 84 disposedabout a shaft 86 which is anchored to the nacelle 18 on one end thereofand pivotably attaching to a first bracket 88 on the other end thereofby means of a pivot 90. The first bracket 88 is pivotably fixed to thenacelle 18 and to a second bracket 92 by means of pivots 94, 96,respectively. The flap actuating arm 48 is also pivotably attached tothe second bracket 92 by means of a pivot 98. A mechanical restraint 99may be included, as shown in FIG. 6.

[0033] In operation, during take-off and landing, the flaps 38 of thevariable area nozzle 30 are in the diverged position, as shown in FIG.2. In the diverged position, the flaps 38 define the opened or enlargedfan exit nozzle area 36. In the diverged position of the flaps 38, theSMA actuator 68 is in deformed shape. When the SMA actuator 68 is in thedeformed shape, the SMA material is at a low temperature martensiticstate. To maintain the SMA actuator in its deformed shape the SMAactuator 68 must remain at a temperature below its phase transformationtemperature or be mechanically restrained.

[0034] Once the plane reaches its cruise mode, the plurality of flaps 38are actuated into the converged or closed position, as shown in FIG. 3.To actuate the flaps 38 into the converged position, the SMA actuator 68is heated above its phase transformation temperature into austeniticstate. The control system 81 applies heat or voltage across the SMAmaterial to heat the SMA actuator 68. As the SMA actuator 68 istransforming from its deformed shape into its parent shape, the SMAactuator contracts, pushing the drive arm 54 outward, as shown in FIG.3. As the drive arm 54 is moved outward, the upstream end of the flapactuating portion 48 and the link 60 are also moved outward, forcing thedownstream end of the flap actuating portion 48 to pivot inward aboutthe pivot 64, thereby moving the flap tip 34 inward to reduce the fanexit nozzle area 36. The four bar linkage 52, as shown in FIG. 4, isconfigured to yield a substantially parallel motion of the SMA actuator68. As the SMA array 72 moves outward, each of the SMA strands 70 isstrained substantially the same amount, resulting in optimized workoutput for all SMA strands.

[0035] Once the flaps 38 need to be placed into the opened position foran aircraft to land, the control system 81 removes the heat from the SMAactuator 68 to allow the SMA material to return to its martensiticstate. Additionally, for expedited transformation of the SMA materialfrom austenitic state to martensitic state, the SMA actuator 68 can beactively cooled. As the SMA material transforms into its low temperaturemartensitic state, the return mechanism 42 overcomes the SMA actuator68, thereby deforming the SMA material, as shown in FIG. 2. To deformthe SMA actuator, the spring 84 acts on the first bracket 88 at pivot 90forcing the pivot 96, joining the first bracket 88 and the secondbracket 92, to rotate radially inward. As the first bracket 88 rotatesabout pivot 94, the upstream end of the flap actuating portion 48 isforced to move radially inward at the pivot 98. When the upstream end ofthe flap actuating portion 48 moves radially inward, the flap tip 34 isforced outward to enlarge the fan nozzle exit area 36 while the drivearm 54 is forced inward, deforming the SMA material.

[0036] Once the SMA material is deformed, the mechanical restraint 99may be employed to ensure that the SMA material remains deformed. Themechanical restraint 99 can be a latch or a pin or any other mechanicaldevice for maintaining the SMA array in its deformed shape.Alternatively, the return mechanism 42 can continue to retain thedeformed position of the SMA. Additionally, the flaps 38 can be latchedor the return mechanism 142 can be latched or restrained.

[0037] Referring to FIG. 7, the variable area nozzle 130, according toanother embodiment of the present invention, includes a return mechanism142 actuated by a secondary SMA actuator 143. The variable area nozzle30 includes an actuating mechanism 140 substantially analogous to thatof the variable area nozzle 30 with the like elements designated by thelike reference numbers preceded by numeral 1. The actuating mechanism140 includes a four bar linkage 152 with a drive arm 154. The drive arm154 has a first surface 155 and a second surface 157 with the firstsurface 155 being engaged by the primary SMA actuator 168 and the secondsurface 157 being engaged by the secondary SMA actuator 143. Thesecondary SMA actuator 143 has a parent shape, shown in FIG. 7, and adeformed shape (not shown). When the secondary SMA actuator 143 is inits parent shape, the primary SMA actuator 168 is in its deformed shapeand when the secondary SMA actuator 143 is in its deformed shape, theprimary SMA actuator 168 is in its parent shape. The secondary SMAactuator 143, similarly to the primary SMA actuator 168, comprises aplurality of SMA strands 170 formed into an array 172.

[0038] Referring to FIG. 8, the secondary SMA actuator 143 is loopedthrough the gas turbine engine to prevent interference with the primarySMA actuator 168.

[0039] In operation, the actuating mechanism 140 operates substantiallyanalogously to the actuating mechanism 40 described above to reduce thefan exit nozzle area 36. The return mechanism 142 is actuated byapplying power or heat to the secondary SMA actuator 143. Once thesecondary SMA actuator 143 is powered or heated, the secondary SMAactuator 143 transforms from its martensitic to its austenitic state,generating a force output to move the drive arm 154 radially inward. Asthe drive arm 154 is actuated inward, the primary SMA actuator 168deforms and the drive arm 154 pulls the flap actuating portion 148inward. As the flap actuating portion 148 is moved inward, the flap tips134 are moved outward, enlarging the fan exit nozzle area 136. Thus, theprimary and secondary SMA actuators 168, 143 cooperate to enlarge orreduce the fan exit nozzle area 136. The primary and secondary SMAmaterials are alternatingly heated (and/or cooled) to transform the SMAmaterial from its martensitic state to its austenitic state (and viceversa). As the SMA material transforms from its deformed shape inmartensitic state to its parent shape in austenitic state, the forceoutput is generated by the primary and secondary SMA actuators 168, 143to move the drive arm 154 outward or inward, respectively, to close oropen the flaps 138, respectively. As the primary SMA actuator 168generates the force output and moves the drive arm 154, the secondarySMA actuator 143 is deformed and vice versa. Additionally, to expeditethe cycle, the SMA actuators 143, 168 can be cooled to hasten thetransformation of the SMA material from austenitic state to martensiticstate. The shape of the drive arm 154 allows the primary and secondarySMA actuators to translate motion while straining each SMA arraysubstantially the same amount.

[0040] The variable area nozzles 30, 130 of the present inventionachieve approximately 20% (twenty percent) change in the fan exit nozzlearea 36, 136. The flaps 38 move approximately two to two and one-halfinches (2″ to 2.5″) inward when actuated into the converged position.Such actuation is highly beneficial during cruise conditions to improvefuel efficiency. However, other systems can be tailored to achievevarious degrees of actuation required for a particular application.

[0041] In order to effect change from the martensitic to austeniticphase, the SMA actuator 68 can be heated by many alternative power orheating sources and methods. For example, the SMA actuator 68 can beheated with hot air or hot water, conduction, convection or radiationheat, electrically, and/or by other means. However, in the preferredembodiment of the present invention, the control system 81 appliesvoltage across the SMA actuator 68 to heat and cause the transformationof the SMA actuator from martensitic state to the austenitic state. Theamount of heat to be applied to the SMA actuator 68 to reach phasetransformation temperature depends on various factors, such as how fastthe SMA actuator must be actuated, overall size of the SMA actuator, andbase temperature of the SMA actuator. Similarly, the SMA actuator 68 canbe cooled by a variety of methods. For example, cool air can becirculated to cool the SMA material.

[0042] The amount of work the SMA actuator performs depends on theparticular number of individual SMA wires 74 forming the SMA actuator 68and the length of these wires. For example, if each individual wire 74outputs X force and the SMA includes nineteen (19) wires, then the totaloutput from the SMA actuator is approximately 19X. Thus, according tothe present invention, various configurations of SMA actuators 68 can beformed to obtain the desired amount of force. In the preferredembodiment of the present invention, the SMA actuator 68 can generateapproximately three-thousand (3,000) inch-pounds of moment about flapaxis of rotation which coincides with pivot 64.

[0043] Additionally, in the preferred embodiment, the plurality of flaps38 are sealed (not shown) therebetween to minimize air leakage.

[0044] The SMA wires 74 can be either pure nickel-titanium (NiTi) alloyor nickel-titanium alloy modified with various other elements (such ascopper (Cu)) or another material that exhibits shape memory effect. Thespecific composition of the SMA can vary in order to optimize propertiesnecessary for the particular application. In the preferred embodiment,the raw SMA wires were manufactured by Raychem Corporation of MenloPark, Calif. However, various other companies also manufacture SMAmaterial.

[0045] The SMA actuators 68 formed from substantially flexible SMAstrands, can be terminated in a variety of different arrangements. TheSMA strands 70 formed into arrays 72 can be also terminated in a varietyof conventional mechanical terminations that are typically used inconventional wire rope making art.

[0046] Referring to FIGS. 9-11, another embodiment of a four bar linkage252 driven by a SMA actuator 268 includes an output arm 254, with theSMA actuator 268 secured thereto, and a fixed arm 262. The four barlinkage 252 also includes a flap 238 and a link 260 connecting theoutput arm 254 and the fixed arm 262 by means of pivots 256, 264,respectively. The SMA actuator 268 has a first end 278 and a second end280, as best seen in FIG. 11, and includes a plurality of SMA units 272that can be formed from either SMA strands or SMA ropes, as discussedabove and shown in FIG. 5. The first and second ends 278, 280 of the SMAactuator 268 are fixed, as shown in FIG. 11. The four bar linkage 252 isconfigured to translate a sweeping motion of the SMA actuator 268 in anon-parallel fashion. The four bar linkage 252, according to thisembodiment of the present invention, can be used with complex geometryof the variable area nozzle or with any other mechanism that requiresnon-parallel motion.

[0047] The major benefit of the variable area nozzle 30, 130 and of thefour bar linkage 52, 152, 252 driven by SMA actuators of the presentinvention is that they are actively controlled and used in multi-cycleapplications to generate significant force output. One major advantageof these mechanisms is relative simplicity and compactness.

[0048] Another major advantage of the variable area nozzle 30, 130 ofthe present invention is that a gas turbine engine can be equipped witha variable area nozzle without incurring a significant weight penalty.The variable area nozzles 30, 130 of the present invention, driven by aSMA actuator, are substantially lighter than existing variable areanozzle configurations. The variable area nozzle 130 having a returnmechanism 142 actuated by the secondary SMA actuator 143 is moreadvantageous and results in even greater weight savings. This advantageof the present invention allows practical use of the variable areanozzle on the gas turbine engines.

[0049] A further major advantage of the present invention is that thevariable area nozzles, driven by at least one SMA actuator, do notrequire extensive maintenance. Unlike existing variable area nozzlesthat include complex mechanisms and are driven either hydraulically orpneumatically, the variable area nozzles 30, 130 of the presentinvention do not include a complex mechanism requiring expensive andtime consuming maintenance.

[0050] A further advantage of the present invention is that the SMAactuator 68, configured from a plurality of strands 72 formed from aplurality of SMA wires 74, will not fail catastrophically under normalfatigue or overload situations. Since the SMA actuator 68 includesmultiple smaller diameter wires, when one or more wires fail, suchfailure will be apparent during visual, electrical or other type ofnondestructive inspection. Thus, when failure in one or several wires isdetected, repair or replacement can be scheduled to avoid subsequentcatastrophic failure.

[0051] While the present invention has been illustrated and describedwith respect to a particular embodiment thereof, it should beappreciated by those of ordinary skill in the art, that variousmodifications to this invention may be made without departing from thespirit and scope of the present invention. For example, although thepreferred embodiment describes the SMA actuator 68 formed into an arrayof SMA strands, the SMA actuator 68 can be formed from a SMA rod ortube. Furthermore, the SMA actuator 68 can be formed from a plurality ofindividual SMA wires that are bundled together in various configurationsof strands or ropes. Additionally, the SMA wires 74 can have a roundcross-section or other shapes of cross-section. Moreover, although theSMA actuator 68 and the primary SMA actuator 168 are heated to close ordeploy the flaps 38 of the variable area nozzle, the SMA actuator 68 andthe primary SMA actuator 168 can be heated to open or place the flaps inthe diverged position. Also, the parent shape of the SMA can be eithercontracted or expanded. Furthermore, the SMA actuators 68, 168 mayinclude multiple terminations and multiple power sources. For example,the SMA actuators 68, 168 can be segmented with each SMA actuatorsegment spanning a half, a quarter or any other portion of the engine'scircumference.

[0052] Additionally, various other return mechanisms can be used todeform the SMA actuator. Also, although the preferred embodiment of thepresent invention is described as having a return mechanism 42 and anactuating mechanism 40 corresponding to each flap 38, each returnmechanism 42 and each actuating mechanism 40 can drive more than oneflap 38. Furthermore, various other configurations of four bar linkages52, 152, 252 are within the scope of the present invention.

We claim:
 1. A gas turbine engine situated about a center axis andenclosed in a nacelle, said nacelle having a trailing end in adownstream portion thereof, said trailing end defining a fan exit nozzlearea, said gas turbine engine comprising: a plurality of flaps each ofsaid flaps having an aerodynamically shaped body having a flap tipcoinciding with said trailing end of said nacelle; a plurality ofactuating mechanisms for driving said plurality of flaps into an openposition and a closed position corresponding to an enlarged fan exitnozzle area and a reduced fan exit nozzle area, each of said actuatingmechanisms being driven by a SMA actuator, said SMA actuator beingalternatingly deformed in its martensitic state and heated to itsaustenitic state to actuate said plurality of actuating mechanisms; anda plurality of return mechanisms associated with said plurality ofactuating mechanisms for deforming said SMA actuator in its martensiticstate.
 2. The gas turbine engine according to claim 1 wherein each saidactuating mechanism comprises: a four bar linkage having an output armand a fixed arm pivotably connecting to a first link and a second linkfor translating a substantially parallel motion, said output arm beingengaged by said SMA actuator for actuating said actuating mechanism todrive said flap.
 3. The gas turbine engine according to claim 2 whereinsaid return mechanism comprises a secondary SMA actuator with saidsecondary SMA actuator engaging said four bar linkage to deform said SMAactuator of said actuating mechanism.
 4. The gas turbine engineaccording to claim 1 wherein each said actuating mechanism comprises: afour bar linkage having an output arm and a fixed arm pivotablyconnecting to a first link and a second link for translating asubstantially nonparallel motion, said output arm being engaged by saidSMA actuator for actuating said actuating mechanism to drive said flap.5. The gas turbine engine according to claim 1 wherein said returnmechanism comprises a spring for deforming said SMA actuator.
 6. The gasturbine engine according to claim 1 wherein said return mechanismcomprises a secondary SMA actuator.
 7. The gas turbine engine accordingto claim 6 wherein said secondary SMA actuator is heated and deformedalternatingly with said SMA actuator of said actuating mechanism.
 8. Thegas turbine engine according to claim 1 further comprising a controlsystem to heat said SMA actuator.
 9. The gas turbine engine according toclaim 1 further comprising a mechanical stop to maintain said SMAactuator in said deformed state.
 10. The gas turbine engine according toclaim 1 wherein said SMA actuator comprises an array formed from aplurality of SMA wires.
 11. The gas turbine engine according to claim 1wherein said SMA actuator comprises an array formed from a plurality ofSMA strands.
 12. The gas turbine engine according to claim 11 whereineach of said SMA strands is formed from a plurality of SMA wires. 13.The gas turbine engine according to claim 1 wherein said SMA actuator isformed from a plurality of SMA strands.
 14. The gas turbine engineaccording to claim 1 wherein said SMA actuator is a rigid SMA member.15. The gas turbine engine according to claim 1 wherein said SMAactuator comprises a SMA strand looped into an array.
 16. The gasturbine engine according to claim 1 wherein said SMA actuator includes aplurality of SMA actuators.
 17. A gas turbine engine situated about acenter axis and enclosed in a nacelle, said gas turbine engine having avariable area nozzle, said gas turbine engine comprising: a plurality offlaps each of said flaps having a flap body having a flap tip defining adiameter of said variable area nozzle, said plurality of flaps having anopen position corresponding to an enlarged diameter of said variablearea nozzle and a close d position corresponding to a reduced diameterof said variable area nozzle; a plurality of actuating mechanisms fordriving said plurality of flaps into said open position and said closedposition; a primary SMA actuator for driving said plurality of actuatingmechanisms, said primary SMA actuator being alternatingly deformed inits martensitic state and heated to its austenitic state to actuate saidactuating mechanism; and a secondary SMA actuator for deforming saidprimary SMA actuator to drive said plurality of flaps into said openposition and said closed position.
 18. The gas turbine engine accordingto claim 17 wherein said secondary SMA actuator is heated and deformedalternatingly with said primary SMA actuator.
 19. The gas turbine engineaccording to claim 17 wherein said primary SMA actuator and saidsecondary SMA actuator are alternatingly heated and cooled.
 20. Amechanism comprising: a four bar linkage having a fixed arm and anoutput arm linked by a flap and a link; and a SMA actuator engaging saidoutput arm, said SMA actuator generating a force output upon transitionfrom a martensitic state to an austenitic state.
 21. The mechanismaccording to claim 20 wherein said four bar linkage is configured forsaid SMA actuator to generate substantially parallel motion.
 22. Themechanism according to claim 20 wherein said four bar linkage isconfigured for said SMA actuator to generate substantially nonparallelmotion.
 23. The mechanism according to claim 20 wherein said SMAactuator comprises an array formed from a plurality of SMA strands. 24.The mechanism according to claim 20 wherein said SMA actuator has afirst SMA end and a second SMA end with said first and second SMA endsbeing fixed.
 25. The mechanism according to claim 20 wherein said SMAactuator is a rope.
 26. The mechanism according to claim 20 wherein saidSMA actuator is a rigid member.